1. Field of the Invention
This invention relates to a process for anodically forming a duplex coating on the surface of a valve metal such as aluminum and products resulting therefrom. More particularly, this invention relates to an anodically formed protective coating on a valve metal surface which comprises a duplex layer of metal oxide directly bonded to the valve metal surface and a functionalized layer of a phosphate ester chemically bonded to the metal oxide.
2. Description of the Related Art
It is well known to anodize valve metals such as aluminum in electrolytes containing acids such as sulfuric, chromic, oxalic and phosphoric acid to achieve a porous metal oxide coating, i.e., a porous aluminum oxide coating, which will protect the metal, provide a decorative appearance or facilitate subsequent adhesive bonding to the metal surface.
U.S. Pat. Nos. 4,388,156, 4,381,226, 4,448,647, 4,399,021, 4,383,897, 4,308,079 and West German Patent 3,305,354 describe processes for treating aluminum with organic and inorganic acids.
In the present invention, it has been discovered that a duplex coating can be applied to a valve metal surface in a single process. The coating comprises a layer of anodically formed valve metal oxide and a layer which is comprised of the reaction product of phosphoric acid esters.